This invention relates to a laser printing system utilizing a high speed multi-beam raster output scanner (ROS) and, more particularly, to the interlacing of consecutive modulated beams from a solid state multi-emitter laser source to reduce the effects of periodic (banding) errors caused within the ROS optical/mechanical system and the photoreceptor drive system.
It is known in the scanning art to utilize a multiple laser beam source for scanning a plurality of lines on a recording medium surface simultaneously with a plurality of laser beam outputs. This technique is finding increasing usage because it substantially increases the effective printing speed of a system compared to the conventional laser ROS printer using a single output laser source. U.S. Pat. No. 4,637,679 discloses a multi-beam scanning system utilizing two semi-conductor laser sources LD1, LD2, which are arranged as shown in FIG. 2. Their output beams are formed into parallel light flux which are polarized perpendicularly to each other. Each beam is independently modulated by signals of the scanning line of an odd number and an even number ,respectively, and it then becomes possible to write information as multiple scanning lines. A similar system is disclosed in an article in APplied Optics, July 1987, Vol. 26, No. 13, by A Arimoto, et al., entitled "Dual Beam Laser diode Scanning Systems". Further advances in the art have led to the laser emitter sources being formed within the same semi-conductor chip. Examples of this type of scan system are found in U.S. Pat. Nos. 4,788,560, 4,796,964, and 4,547,038. In U.S. Pat. No. 4,788,560 there is disclosed a scanning system wherein the scanning beam outputs are generated by a plurality of laser sources. The invention described therein is directed to a time sharing method for controlling the light amounts of the respective beams. U.S. Pat. No. 4,796,964 discloses a monolithic two beam laser emitter shown in FIG. 1. The beams from the two emitters are collimated and then directed by a rotating polygon mirror assembly to form multiple scan lines on a photosensitive drum. The invention therein is directed to an operational sequence intended to solve the problem of optical interference between the emitter outputs by controlling the ON-OFF state of each emitter. U.S. Pat. No. 4,547,038 also discloses a multi-emitter semi-conductor laser unit. The invention claimed therein is directed towards the optical system required to collimate and spatially separate the light beam outputs.
A specific problem in the multi-beam type of printing system, not addressed in the above-discussed patents, is that of banding in the output print. Banding is the appearance of horizontal or vertical stripes of lighter or darker density which usually traverse the full width or length of the image being formed in the photosensitive image plane. It is a problem particularly with scanning exposure sources which involve repeatedly traversing an information carrying, intensity modulated, beam of actinic radiation over the surface of the photsensitive medium. This type of system includes those which are contemplated by the present invention. Of particular interest for purposes of the proposed invention is the horizontal banding component. Horizontal bands, that is, bands extending transversely to the direction which the imaging medium moves, are usually due to drive irregularities such as gear and axis errors or vibration of any of the optical components (mirror, lenses), or vibration of the image medium (e.g., the photoreceptor drum), or wobble in the scanning polygon mirror assembly. These vibrational phenomena cause the scan lines to have a periodic spacing error, and it is this error that results in the horizontal bands on the final print. The problem is present in light lens scanning systems and in light valve printers of the type disclosed in U.S. Pat. No. 4,797,694. However, the problem has not been considered, nor solutions obtained, to compensate for banding effects in a multi-beam type of laser scanning printer. U.K. Patent No. 2,069,176 discloses a multiple beam scanning technique which allows beams adjacent at the source to scan nonadjacent lines on the photoconductor, but does not consider the banding problem in conjunction with the scan technique.
The present invention is, therefor, directed to a multi-spot scanning printer which includes a monolithic semi-conductor laser source having a plurality of emitter output points. It has been found that the banding phenomena described above is dependent upon the beam spacing and interlacing at the print medium and, more particularly, that there are specific interlacing values expressed in one embodiment as a particular cosine function, whereby banding is greatly reduced. Still more particularly, the invention is directed toward a multi beam scanning apparatus comprising a laser light source for generating a plurality of light beam outputs, optical scanning means for receiving said output beams, and directing the beams as a plurality of scanning beams to simultaneously form a plurality of raster scanned lines on the surface of a photosensitive image plane, the invention characterized by each consecutive line being formed by non-consecutive scans, the lines being interlaced by an interlacing order greater than one, horizontal banding in the output scan lines being increasingly minimized as the interlacing order increases.